Oxyfuel cutting, plasma cutting, and laser cutting are three principal methods used to thermally cut a metallic workpiece. Oxyfuel cutting is mainly used to cut mild steel where the benefits of the exothermic burning reaction of oxygen and iron are used to do the cutting. In this process, the reaction rate and the resulting cutting rate is determined by the diffusion rates of the reactants and the shear of the gas jet on the liquid metal to remove it from the cut. For cutting a mild steel workpiece having a thickness in the range from about 10 mm to about 12 mm, typical cutting speeds range from about 0.5 to about 1.5 meters/minute. Kerf widths vary from about 1 mm to greater than about 3 mm.
In plasma cutting, the energy used to cut a workpiece is supplied by an electric-arc-heated plasma gas jet which is directed toward or brought in contact with the workpiece. The plasma cutting technique works on all types of electrically-conductive materials and, therefore, has a wider application range than oxy-fuel cutting. Typical plasma arc temperatures are greater than 6000° C. During plasma cutting, metal from the workpiece is removed from the kerf by the shear of the very high velocity plasma-arc jet. Typical cutting speeds for plasma cutting are greater than those of oxyfuel cutting A typical cutting speed for cutting ½″ mild steel with oxy-fuel is about 16 inches/min; whereas a 200 Amp plasma system would typically cut that same size material at 80 inches/min. Kerf widths for plasma cutting are about the same size or larger then those for oxyfuel cutting. The relatively large kerf width has an adverse influence on the precision of the plasma cutting process.
In laser cutting, the energy used to cut a workpiece is supplied by a laser beam directed toward or brought in contact with the workpiece. Material is removed from the kerf by the shear from an assist gas jet directed into the kerf. In laser cutting, kerf widths are narrow. Kerf widths typically range from about 0.15 mm to about 0.5 mm. These narrow kerf widths consequently yield higher precision cutting than is possible with either oxyfuel or plasma cutting. However, in laser cutting, it becomes difficult to remove the molten metal from the kerf as the workpiece thickness increases. This limits the cutting speed and the maximum thickness capability for laser cutting. It is believed that the reason for this limitation is that the high gas velocity required to achieve sufficient gas shear creates supersonic shock waves a few millimeters into the kerf. These shock waves limit the gas shear and its ability to remove metal.
A fourth method for thermally cutting a workpiece is disclosed in U.S. Pat. No. 5,288,960. In this thermal-cutting method, a high temperature liquid metal stream is directed at and impinges on the workpiece. The temperature of the stream exceeds the melting temperature of the workpiece. The problem of removing the molten metal from the kerf because of limited gas shear encountered in laser cutting is thus eased by using a medium (i.e., liquid) with a higher specific density. Compared to laser cutting, higher cutting speeds, thicker workpiece capability, and equivalent high precision cuts can be realized with this liquid-metal-stream cutting approach. However, because of the need to supply a high speed liquid stream to the workpiece, at a temperature greater than the workpiece melting point, this approach has been limited in its use for cutting certain metal. The material requirements for a high temperature, high pressure, liquid containment vessel severely limits the practicality of cutting metals such as aluminum, stainless steel and mild steel, where typical melting temperatures are 660° C., 1400° C. and 1550° C., respectively.
Several methods are used to thermally weld a workpiece. The most widely used welding processes use heat sources to cause localized heating of two or more workpieces, allowing them to melt and flow together. A filler metal generally is added to the weld area in order to supply sufficient material to fill the joint and to increase mechanical strength. For example, a fillet weld generally forms a radial sector of additional material over a weld groove when completed. When the welding process is progressing, a molten pool of workpiece forms and a filler material is moved along the welding front. When the welding heat source is removed, the molten metal solidifies, and the parts are fused or welded together. Common heat sources used to provide heat to melt the workpieces are DC or AC electrical arc, oxy-fuel gas flame, and laser beam.